The Global Positioning System (GPS) has been a revolutionary technology that has made a significant impact on people's lives. Using a smartphone, a person is easily able to navigate anywhere on earth. The only drawback is that GPS does not work well indoors due to the weak signals emitted from the GPS satellites. There is, thus, a need for a positioning system to complement GPS and provide the ability to locate smartphones as well as other assets indoors. Significant research and development has been conducted in use of location systems based on radio frequency (RF). State of the art technologies use a combination of radio waves emitted from WiFi access points and/or Bluetooth beacons. In addition, if the device to be located has an accelerometer and gyroscope, dead reckoning can be performed when these radio signals are temporarily not present. Although this solution works in certain cases, there are several disadvantages with this approach. The first is that indoor environments tend to reflect radio waves creating multiple radio paths to a receiver. There are three typical approaches used to locate devices using radio waves. These are time of arrival (ToA), angle of arrival (AoA) and received signal strength (RSS). The multipath problem impacts the positioning accuracy attainable when using any of the three above positioning techniques. The multipath problem causes significant problems for radio based positioning networks since radio waves travel at the speed of light. When multipath reflections arrive at a radio receiver, they arrive at almost the same time, making it very difficult to resolve the paths.
One way to improve the positioning accuracy is to place more radio beacons in the environment. This approach while improving performance is more expensive. It is also more difficult to calibrate the entire network, since the location of all of the beacons needs to be known before the network can be used to position other devices.
Another promising technology that has been proposed for indoor positioning is using acoustic based measurements. Acoustic based positioning is promising since acoustic waves in air travel at only 340 m/S, approximately a million times slower than radio waves. This makes it much easier to obtain precise positioning and resolve multipath. In this approach, it is typical for several acoustic emitters, each emitting different signals to be placed at known locations in a room. It is typical for frequencies above 20 KHz to be used which is above the audible range. A microphone is used on the receiving end to measure the time of flight from each of these speakers. Using the time of flight and known speaker positions, the receiving device can figure out its position. When each speaker emits an audio signal, it can also emit a radio signal indicating when the speaker started emitting the audio signal. The receiving device can use the difference in arrival time of the radio signal and audio signal to compute the times of flight of the audio waves. There are three drawbacks with this approach. The first is that a significant number of speakers distributed over an area are typically required to cover the area since a device to be positioned needs to hear the sound signals from at least three or four different speakers. The second problem is that sound waves, especially higher frequencies, tend to not be able to penetrate walls and other obstructions. The 3rd problem, which is shared with the RF based approach, is that all speaker positions need to be calibrated before they can be used for positioning.